Optimize loads from variables that might be shadowed by variables

Optimize loads from variables that might be shadowed by variables
introduced by eval.

In the cases where calls to eval have not introduced any variables, we
do not need to perform a runtime call.  Instead, we verify that the
context extension objects have not been created and perform a direct
load.

Not implemented for ARM yet and the scope resolution code could use
some better abstractions.  I'd like to do that in a separate
changelist.
Review URL: http://codereview.chromium.org/20419

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1298 ce2b1a6d-e550-0410-aec6-3dcde31c8c00